This invention relates to thermostats for electric heating or cooling apparatus. The invention is discussed in connection with so-called line voltage thermostats, wherein the load current required by an electric heater or a room air conditioner is carried by the switch that forms part of the thermostat, but it applies equally to thermostats of other types or of different constructions.
A well-known type of line voltage thermostat includes an ambient temperature sensing bimetal formed of an elongated sheet or plate of sufficient width for stiffness. The bimetal element is pivotally mounted near one end from a base plate and carries an adjustment screw in its mid section which bears against the spring-loaded plunger of a snap-switch which is also carried by the base plate. A rotatable cam bears against the bimetal element near the other end in opposition to the force exerted against the screw in the bimetal element by the plunger and the spring of the switch.
The set-point temperature of the aforesaid thermostat is determined by the rotational position of the cam. Where the thermostat has an adjustment screw, it is calibrated by turning the screw into or out of the bimetal element while the plunger of the switch is spring-biased against the screw. It is usually desired to calibrate the thermostat so that the switch will be actuated when a pointer on the cam or on a knob affixed to it points upright or to a "normal" position and the temperature rises to approximately 70.degree. F. The adjustment of the calibrating screw is usually performed by the manufacturer and then locked or cemented against further rotation.
The calibration of a thermostat usually should not be disturbed or changed after it has been adjusted and fixed. The thermostat is commonly mounted to an electrical box that is set into a wall, for this purpose using screws extending from the base plate of the thermostat to threaded ears in the box. The back or mounting plane of the thermostat is pressed against the surface of the wall when the screws are tightened. The tightening of the mounting screws for attaching the thermostat to a wall or other mounting surface can disturb its calibration by stressing and thereby physically distorting the base plate of the thermostat.